High frequency amplifier



Sept. 19, 1939.

v. D. LANDON HIGH FREQUENCY AMPLIFIER Filed Dec. 15, 1936 FREUENC r 3nvento1: .D.

(Ittorneg Patented Sept. 19, 1939 HIGH FREQUENCY AMPLIFIER Vernon D. Landon, Haddonfield, N. J. assignor to Radio Corporation of America, a corporation of Delaware Application December 15, 1936, Serial No. 115,915

6 Claims.

My invention relates to high frequency amplifiers and particularly to amplifiers of the resistance coupled type.

In all resistance coupled amplifiers the tube capacities limit the frequency range or band width of the amplifier. This limitation may be serious in a television receiver in which a resistance coupled amplifier is employed for the amplification of picture signals. If the gain of the amplifier drops off too soon at the high frequency end of the response curve, the received picture will be lacking in the desired detail.

Accordingly, an object of my invention is to provide an improved high frequency amplifier.

A further object of my invention is to extend the response curve of a resistance coupled amplifier at the high frequency end.

A still further object of my invention is to provide a resistance coupled amplifier in which the product of. gain multiplied by band width is greater than that previously obtainable.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing, in which Figure 1 is a circuit diagram of an amplifier embodying my invention, and

Figure 2 is a group of curves which are referred to in explaining the invention.

Referring to Fig, 1, the amplifier comprises amplifiers l and II, each of which preferably is of the screen grid type. The tube Iil includes an indirectly heated cathode l2, a control grid I3, a screen grid I4 and an anode I6. The control grid may be biased negatively a suitable amount by means of a self-biasing resistor l'l,v

the resistor ll being shunted by the usual bypass condenser Ill.

The tube II also includes an indirectly heated cathode [9, a control grid 2!, a screen grid 22 and an anode 23. The grid 2| may be negatively biased by means of a self-bias resistor 24 shunted by a by-pass condenser 26, the bias voltage being applied through a grid resistor 21.

Suitable voltages are applied to the screen grids I4 and 22 from any suitable source as indicated on the drawing.

In the specific embodiment of the invention illustrated, voltage is applied from a suitable source such as battery I to the anode [6 of the tube Ill through a plate resistor 28, an inductance coil L1 and another inductance coil L2. As will be explained later, the coil L2 may be omitted in some cases.

The plate l6 of the tube I0 is coupled to the grid 2| of the tube ll through the coil L2, through another inductance coil L3, and through a coupling condenser '29.

The effect of the coils L1, L2 and L3 will be seen by referring to the curves in Fig. 2. The curve A shows the frequency response characteristic of the amplifier with the coils L1, L2 and L3 omitted. By adding the coil L1 the frequency response is extended as shown bythe curve B, the coil L1 increasing the external plate impedance at the higher frequencies.

By adding the coils L2 and L3, thetwo coils having approximately the same inductance, and having less inductance than coil L1, the response curve is extended at the high frequency end, as shown by the curve C. The coils vL2 .and L3 are given the proper inductance values to make them resonate with the plate cathode capacity Cp of tube Ill and any distributed capacity in parallel therewith and with the grid-cathode capacity Cg of tube H and any distributed capacity in parallel therewith. The frequency at which this circuit resonates is indicated by the peak in the curve C at the high frequency end,

If coils L2 and L3 are of equal value, the plate resistor 28 does not damp the circuit C L2, L3, Cg, since the top of L1 is connected at a nodal point in this circuit. It may be noted that the nodal point referred to is with respect to the voltages built up in the resonant circuit, the driving voltage supplied from tube Ill being ignored as it is relatively small.

In order to keep the resonate peak of the curve C from rising to an excessively high value, a certain amount of damping in the circuit is generally desired. This may be provided by making coils L2 and L3 of unequal values, whereby the plate resistor 28 damps the circuit, or damping may be provided by resistors 31 and 32 connected in shunt to the coils L2 and L3. respectively.

By omitting either coil L2 or coil L3 and by increasing the inductance value of the remaining coil to produce resonance at the same frequency, the resonance peak is reduced in amplitude by the action of the plate resistor 28 as shown by curve D.

It may be pointed out that the use of the coil L1 makes it possible to resonate coils L2 and L3 with the tube capacities at a higher frequency than would otherwise be possible and thus extend the frequency response as there would be a pronounced dip in the curves C and D if the coil L1 were omitted.

I claim as my invention:

1. A high frequency amplifier comprising an amplified, a source of plate potential, means for applying potential from said source to the anode of said first tube, said means comprising a plate resistor and another inductance coil connected in series between a point on said connection and said source of plate potential, said last mentioned coil being shunted only by its distributed capacity and having the proper inductance to extend the high frequency response of said amplifier, said last-mentioned coil and said distributed capacity being nonresonant within the frequency range of said amplifier.

2. The invention according to claim 1, characterized in that said first mentioned inductance coil has a damping resistor connected thereacross.

3. A high frequency amplifier comprising an amplifier tube having a cathode, a control electrode and an anode, said tube having anodecathode capacity, a second amplifier tube having a cathode, a control grid and an anode, said sec- ,ond tube having control grid-cathode capacity, a connection between the anode of said first tube and the control grid of said second tube, said connection including two inductance coils connected in series anda plate resistor and a third inductance coil connected in series between a source of plate potential and a point on said first mentioned connection intermediate said first and second coils, said first and second coils having such inductance values that they resonate with said tube capacities and any distributed capacities in parallel with said tube capacities at the high frequency end of the frequency range to be amplified.

4. The invention according to claim 3, characterized in that said two series connected inductance coils are of unlike value, whereby the plate resistor damps the resonant circuit.

5. A high frequency amplifier comprising an amplifier tube having a cathode, a control electrode and an anode, said tube having anodecathode capacity, a second amplifier tube having a cathode, a control grid and an anode, said second tube having control grid-cathode capacity, a connection between the anode of said first tube and the control grid of said second tube, said connection including two inductance coils connected in series, and a third inductance coil connected in series with a plate resistor between a source of plate potential and a point on said first mentioned connection intermediate said first and second coils, said first and second coils having approximately the same inductance values, and the combined inductance being such that they resonate with said tube capacities and any distributed capacities in parallel with said tube capacities at the high frequency end of the frequency range to be amplified, said third inductance coil having an inductance which is greater than "the inductance of either of said other coils and of such value that it extends the high frequency response of said amplifier.

B. The invention according to claim 5, characterized in that at least one of said series connected inductance coils has a damping resistor connected thereacross.

VERNON D. LAN'DON. 

